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Powdery Mildew Alert

Posted by Lauren Barker- Horticulture Specialist on 01 August 2019 in Trees, Plants & Ornamentals
Powdery Mildew Alert

The current hot and dry weather is creating the perfect conditions for powdery mildew on ornamentals and garden plants; however, your protected crops are not safe either!

What is Powdery Mildew?

Powdery mildew is a fungus. There are 5 main tribes of powdery mildew and under these fall different genera and species. So, for example Podosphaera pannosa is the specific fungus which grows on roses. The most common method of disease spread is by fungal spores being carried in the air from plant to plant. In cooler conditions, the spores will live for longer periods of time (up to 3 months) but in warmer conditions they will be killed quite quickly. We can also spread the fungus on our hands and tools. When conditions are humid, the spores will germinate very quickly. This means other plants and other parts of the same plant will become infected rapidly.

The disease cycle can take around 5 days under humid conditions. Powdery mildew likes warm, dry days and cooler nights that still have high humidity. Growers find that when the temperature is around 21oC – 22oC spores will germinate rapidly. Humidity of around 100% also encourages the spores to germinate. If the weather deteriorates and the leaves of the plant become wet, it is a lot less likely to spread, however when the leaf dries out again, the spores will continue to germinate.

Powdery mildew will live through the winter in the buds and stems of hardy plants. As soon as the temperatures rise in Spring, the leaves of hardy stock will open, and the spores will be available to spread when conditions are favourable.

Which plants are affected?

Some plants are more susceptible then others. It is always useful to know which ones, so you can monitor them to catch the problem early or use preventative sprays.

Strawberries, gooseberries, oaks, lonicera (honeysuckle), clematis, roses, delphiniums, lupins, pansy, petunia and forget-me nots are some of the plants which are most susceptible to powdery mildew. Whether crops are field grown, or pot grown, they can still be affected.

What does it look like?

If your plants have contracted this disease you will be able to see white powdery patches of fungus either on the upper or lower surface of the leaves. It can also be seen on flowers, stems and fruit. Leaves can also become distorted, wrinkled or curled. This is particularly common in rose leaves. You will usually see the disease on new growth, but this doesn’t mean that old growth won’t be affected; we recently found some petunias on a site where the base of the plants was noticeably more affected than the new growth. The leaves will eventually become yellow and fall off. The leaves on some shrubs can turn red/brown underneath the powdery mildew. This is common in roses and rhododendrons. For poinsettias, the mildew predominantly grows on the underside of the leaf and creates a yellow spotting on the surface of the leaf.

What control can be used?

Depending on what you are growing, resistant cultivars are available. It is best to regularly monitor crops to look for early signs of powdery mildew. Walking crops is an invaluable way of keeping pest and disease under control.

When propagating plants, it is best practice to use plants that are healthy and pest and disease free. If you are taking cuttings or splitting herbaceous perennials, ensure that the plants do not have powdery mildew or any other disease.

I recommend keeping growing areas weed free as powdery mildew will happily live on weeds around the site and can easily spread onto crops.  I would also remove any waste plant material as resting spores will survive on this, particularly during autumn leaf fall.

I would urge you to space your plants well to create good airflow and reduce the humidity around each plant. This can be done in glass houses or polytunnels using fans or opening vents or by simply not overcrowding a bed or border. I would advise irrigating your plants early in the day- they are likely to dry out much more quickly.

Fungicides can be used in a spray programme. It is important to begin spraying before the disease is present. Preventative fungicides can be used to keep the disease from damaging crops. It is important to use fungicides with different modes of action to avoid a build-up of resistance and look at spraying more often when the conditions are favourable for the spread of spores. New shoots are particularly susceptible to powdery mildew so they may need to be sprayed more often.

Biological Products:

- AQ10 (Ampelomyces quisqualis) – this is specific to powdery mildew

- Amylo (Bacillus amyloiquefaciensis) – a good broad spectrum fungicide for a wide range of crops including edibles and ornamentals

Specialist Control:

- Potassium Bicarbonate food grade – this has contact fungicidal activity but with no MAPP number.

- Karma (potassium hydrogen carbonate) – contact fungicidal activity with approvals for edible crops.

- Amistar (Azoxystrobin) – systemic broad spectrum fungicide

- Frupica (Mepanipyram) – contact protectant fungicide for outdoor and protected ornamentals and edibles

- Stroby (Kresoxim-methyl) – systemic, protectant fungicide for some edible crops and ornamentals

- Switch (Cyprodinil & fludioxinil) Systemic

- Nimrod (Bupirimate) – Systemic

- Signum (Boscalid & Pyraclostrobin) - Systemic and protectant

- Nativo (Teboconazole & Trifloxystrobin) – Protectant broad spectrum for use in edibles and ornamentals.


If you have any queries about powdery mildew or need help identifying a disease, as always, please don’t hesitate to get in touch.